1. Field of the Invention
This invention relates to differential laser gyros, and particularly to a ring laser configuration in which readout of the laser signals is accomplished by an optical system fixedly mounted on the outside of the block which contains the ring laser. Stability and accuracy of the readout as well as ease of alignment of the readout system is enhanced.
2. Description of the Prior Art
Ring type lasers, employing electromagnetic traveling waves at optical frequencies in a clockwise and anticlockwise direction about a closed path in a principal plane, have been utilized to sense rates of angular rotation in a manner similar in function to the electromechanical gyro. When the laser gyro is physically rotated, more time is required for a traveling wave to complete the optical path in the direction of rotation, and less time is required for a traveling wave to complete the optical path in the direction opposite to the direction of rotation. Thus, if the laser is rotating in the same direction as the clockwise beam, the frequency of the clockwise beam will appear to be less than the natural frequency of the laser, whereas the frequency of the anticlockwise beam will seem to be higher. The difference the two frequencies is a function of the rate of rotation of the gyro.
The two mode laser gyro as described heretofore must be biased to prevent locking between the two optical frequencies at low angular rates. The differential laser gyro comprises two such two mode gyros in the same cavity, biased by the same element but in an opposite sense with respect to rate. When the differential output between the two gyros is measured, rate output is additive resulting in doubled sensitivity with first order bias cancellation. A differential laser gyro system of this type is given the acronym DILAG, and is described in commonly owned U.S. Pat. No. 3,862,803 to which reference may be made for details of the DILAG.
A source of error encountered in the prior art is the optical readout from the laser gyro. Since the gyro operates on the differences in optical frequencies between two pairs of oppositely rotating beams, and since the pairs of beams are of different polarizations and are colinear in their lasing path, it is necessary to extract from the lasing path a portion of each beam and to combine similarly polarized portions of the two beams in a manner that will retain the desired information without at the same time introducing errors in the output. A particular difficulty which occurs is that a right circularly polarized beam becomes left circularly polarized, and vice versa, upon reflection from a mirror, resulting in complexities in designing an accurate readout system.
One prior art approach to obtaining readout from the gyros uses the reflections from the angularly disposed faces of a wedge shaped modulator, the two reflected beams being directed by mirrors disposed on posts within the ring gyro block, combined in a beam splitter and thereupon directed to two separate detectors, one detector responding to the clockwise gyro frequency and the other detector responding to the anticlockwise gyro frequency. This approach, while adequate, is subject to errors introduced by mechanical and thermal instability of the post mounted mirrors. Differential motion of any mirror or beam director, and including the beam splitter which combines the beams, produces a doppler shift in the beam frequency which is seen at the output as a rate signal, and is a source of error. Any angular misalignment between the two beams produces an amplitude change at the detector which also produces a frequency shift in the output and is another source of error. An additional error is produced by any strain, misalignment, or other thermal, mechanical or electromagnetic effect which introduces polarization anisotropies differentially into the beams, causing cross talk between the two gyro signals.
In order to properly align the readout beams to intersect at the beam splitter, it is necessary to mount the reflecting mirrors on mounting devices having one or more degrees of angular freedom, and in some cases having at least one degree of translational freedom, while the beam splitter itself required freedom about two axes, the mechanical mountings adding to the instability of the system in response to mechanical shock or thermal distortion. In the prior art the reflected readout beams are propagated in the same plane but slightly removed from the path of the propagating laser waves, and the readout optics are constrained in location in order to prevent undesired interaction between the reflected readout beams and the laser gyro itself.